The invention relates to a viscous coupling with two parts which are rotatable relative to one another around a common longitudinal axis. These two parts form an annular chamber which is filled with a highly viscous fluid. First coupling plates are connected to the first one of the parts for rotation therewith. Second coupling plates are connected to the second one of the parts and both sets of plates are arranged so as to alternate in the longitudinal direction. The second coupling plates are also arranged so as to alternate with clamping rings in the second one of the parts, and the clamping rings, in turn, are connected to the second one of the parts for rotation therewith. An axial adjusting device acts on the second coupling plates and on the clamping rings and axially locks the same relative to one another and to the second one of the parts.
It is known to use a viscous coupling in the driveline of a motor vehicle with a permanently driven first axle and an optionally driven second axle, wherein the parts of the coupling are connected to an input end and to an output end in a driveline leading to the second axle.
In the above respect, it is known to combine the viscous coupling with a freely controllable switching device which, by means of an axial adjusting device, connects one set of coupling plates alternating with clamping rings in the respective part of the coupling to the part of the coupling or disconnects it therefrom.
DE 40 36 230 C2 and DE 41 07 240 C2 describe the operational parameters that can be freely selected and on which such switching operation depends. Several proposals have already been made for designing such an adjusting device.
It is also known to combine the viscous coupling with a free-wheeling assembly which has a locking effect if the input end leads relative to the output end. The viscous coupling disconnects if the output end leads relative to the input end when the vehicle moves forwards. This is disadvantageous in that, during reversing, i.e. when the direction of rotation is reversed, the free-wheeling assembly disconnects the viscous coupling, and even if there is no grip at all at the first axle, the second axle is not connected. To overcome this disadvantage, it is necessary to provide an additional switching device by means of which the freewheeling assembly can be bridged during reversing.
DE 197 30 714 A1 proposes a power transmitting unit with two coupling parts which are rotatable relative to one another, in the form of a friction coupling wherein a first pressure force generating unit for actuating the friction coupling is formed by a fluid pulse device and a second pressure force generating unit for actuating the friction coupling is formed by a ball/ramp assembly with two ramp discs which are rotatable relative to one another. The ball/ramp assembly is put into effect by braking one of its two ramp discs rotating together with one of the coupling parts. This braking action is achieved by a further friction coupling which is actuated in a contact-free way by an electromagnet which is operative between the brakable ramp disc and the other one of the coupling parts.
DE 199 18 411 A1 discloses a viscous coupling for transmitting power between two coupling parts which are rotatable relative to one another. The viscous coupling is provided with two sets of viscous plates which are arranged in an annular chamber filled with a viscous fluid and of which the one set can be coupled by means of coupling plates to the associated coupling part and uncoupled therefrom. The pressure force applied to the coupling plates is generated by a ball/ramp assembly with two ramp discs which are rotatable relative to one another. The ball/ramp assembly is actuated by one of its two stationary ramp discs which is rotated by an electric motor.
DE 199 02 388 A1 proposes combining a viscous coupling with a friction coupling for transmitting power between two coupling parts which are rotatable relative to one another. The viscous coupling is provided with two sets of viscous plates which are arranged in an annular chamber filled with a viscous fluid, with each set being associated with one of the coupling parts, wherein one of the sets is held in a plate carrier which can be coupled to, and uncoupled from, the associated coupling part. The plate carrier can be coupled to, and uncoupled from, the respective coupling part by an auxiliary coupling. The friction coupling and the auxiliary coupling can be actuated by a ball/ramp assembly which comprises two ramp discs rotatable relative to one another and which is actuated by rotating one of its two stationary ramp discs by means of an electric motor.
DE 196 50 039 A1 proposes a coupling unit which consists of two coupling parts which are rotatable relative to one another, with a friction coupling being arranged between the two coupling parts, and which is actuated by a ball/ramp mechanism. The ball/ramp mechanism comprises two ramp discs which rotate with one of the coupling parts and of which one can be braked by a viscous coupling. The viscous coupling comprises an annular chamber which is filled with a viscous fluid and which contains two sets of viscous plates, each having a plate carrier. One of the plate carriers of the viscous coupling is connected to the brakable ramp disc and the other plate carrier of the viscous coupling is connected to the other one of the coupling parts.
It is the object of the present invention to provide a coupling of the type mentioned above which is characterized by a high degree of operational safety and wear resistance.
The objective is achieved in that the axial adjusting device consists of a ball/ramp mechanism with a pressure ring held in one of the parts for rotation therewith and which is axially displaceable. A freely rotatable setting ring which can be braked relative to the other one of the parts, has balls arranged between both rings, which balls are held in ramp-like circumferential grooves in ring faces facing one another. A brake assembly for the setting ring comprises a second viscous coupling having a second annular chamber which is filled with a highly viscous fluid and in which third coupling plates are connected to the setting ring for rotation therewith. Fourth coupling plates are connectable to the other one of the two parts, and arranged so as to alternate in the longitudinal direction wherein the fourth coupling plates are arranged so as to alternate with second clamping rings in said one of the parts. The clamping rings, in turn, are connected for rotation with one of the parts and have a second axial adjusting device for the setting ring of the brake assembly.
A coupling designed in the above way constitutes a viscous coupling which is almost wear-free and which comprises an adjusting mechanism which is also characterized by a viscous coupling forming a braking device for a ball/ramp mechanism. The second viscous coupling is adjusted in a contact-free and low-wear way via electromagnetic forces. In both viscous coupling units, mechanical friction only occurs between the clamping rings and the coupling plates controlled thereby, with the viscous fluid inside the annular chambers, minimizing the amount of wear. The actual coupling forces are generated by fluid shear. The ball/ramp mechanism itself is also characterized by low wear. Selecting a steep ball ramp angle ensures that the viscous coupling opens quickly. This is frequently required in order to ensure that the viscous coupling in the longitudinal driveline is compatible with anti-lock braking systems and driving stability systems.
Alternatively, it may be provided that the first part is formed by a hub and the second part by a housing or that the first part is formed by a housing and the second part by a hub. In a further improvement the pressure ring is connected to the hub for rotation therewith and the setting ring can be braked relative to the housing. According to another improvement the second axial adjusting device may comprise an electromagnetically displaceable pressure plate in the second annular chamber. Herein, preferably, a magnetic coil of the second axial adjusting device is positioned outside the second annular chamber. The two annular chambers are hydraulically connected to one another.